The invention relates to a titanium-silicalite molecular sieve and the method for the preparation of the same, specifically to a five-member ring titanium-silicalite molecular sieve with MFI structure (TS-1) and the method for preparation of the same.
A crystalline titanium-silicalite molecular sieve is a novel heteroatom substituted silicalite molecular sieve, which was first reported in 1980s"". Now, a variety of titanium-silicalites have been reported, including TS-1 with MFI structure, TS-2 with MEL structure and TS-48 with a larger pore system. These molecular sieves can be applied to catalytic oxidation of different organic substrates, for example, epoxidation of olefins, hydroxylation of aromatics, oximation of cyclohexanol and oxidation of alcohols, and exhibit an excellent reactivity and selectivity in these catalytic oxidations. These crystalline titanium-silicalites used as redox molecular sieve catalysts have a prosperous future for applications in some industrial processes.
TS-1 molecular sieve is a synthetic, crystalline porous material having a structure similar to ZSM-5, obtained by substituting titanium for a partial silicon in the skeleton, which exhibits an excellent catalytic reactivity and selectivity in different oxidations attributed to coordination of the catalytic oxidation property in titanium and shape selectivity effects in ZSM-5 structure. Compared with the traditional oxidation reactions, H2O2 as the oxidant in all the oxidations catalyzed by TS-1 has the advantage of giving environmentally benign water as its by-product and operating simply. Hence, TS-1 makes it possible to develop new industrial processes.
Marco Taramasso et al first disclosed a process for synthesizing TS-1 in 1981 (GB 2,071,071A, U.S. Pat. No. 4,410,501). In their report, the preparation of TS-1 is based on the initial formation of a reaction mixture containing a silica source, a titanium source, an organic base (RN+) and/or an alkaline oxide(Men/2O), followed by hydrothermal crystallization in an autoclave at 130-200xc2x0 C. for 6-30 days. The final product TS-1 is obtained after filtration, washing, drying and calcination. The silicon source used is selected from tetra-alkyl ortho-silicate, or colloidal SiO2, or an alkali metal silicate, and the titanium source used is selected from hydrolyzable titanium compounds, preferably Ti(OC2H5)4. The organic base used is preferably tetra-propyl ammonium hydroxide. The reaction mixtures generally show a composition range in mol % as follows:
Thangaraj et al. indicated that titanium-content in the skeleton of the TS-1 molcular sieve synthesized by the above process was very low, and disclosed a TS-1 molecular sieve synthesis method for effectively increasing titanium content in the skeleton of the synthesized TS-1 molecular sieve(Zeolite, 1992, Vol. 12, p943-950). It was said that, by this method, the value of Si/Ti in the molecular sieve prepared by Taramasso""s method could be reduced from 39 to 20. Thangaraj""s method comprises: adding an appropriate amount of an aqueous solution of tetrapropyl ammonium hydroxide (TPAOH) into a tetraethyl silicate solution with stirring for a certain period of time to get the solution dissolved throughly, then adding an isopropanol solution of tetrabutyl titanate slowly under viogrous stirring to obtain a clear liquid mixture(said solution must be added slowly dropwise to prevent the formation of white TiO2 precipitate due to quick hydrolysis of tetrabutyl titanate); after stirring for 15 minutes, adding another appropriate amount of an aqueous TPAOH solution slowly, then displacing alcohol in the reaction mixture under 75-80xc2x0 C. for 3-6 hours, and afterwords transferring the mixture into an autoclave to undergo hydrothermal crystallization under 170xc2x0 C. for 3-6 days, and after drying to obtain the TS-1 molecular sieve. In the process, the reaction mixture shows a composition in molecular ratio as follows;
SiO2:(0.01-0.10)TiO2:0.36TPAOH:35H2O
Du et al in CN1167082A discloses a method for preparation of a TS-1 molecular sieve, comprising: dissolving a titanium source in an aqueous TPAOH solution, and mixing with solid silica gel pellets homogeneously to obtain a reaction mixture, then undergoing hydrothermal crystallization in an autoclave under 130-200xc2x0 C. for 1-6 days, and afterwards filtrating, washing, drying and calcining the mixture by conventional processes.
The above-metioned methods in the prior art for synthesizing the TS-1 molecular sieves have drawbacks mainly in that, in the course of process, a relatively large portion of stagnant Ti is formed as ex-skeleton Ti remaining in the pore channels of the molecular sieves, this portion of ex-skeleton Ti cannot play an effective role in the catalytic oxidation, but will cause decomposition of the oxidant(H2O2). Consequently, the TS-1 molecular sieves prepared by the above methods exhibit low catalytic oxidation activity, and moreover, due to unstable content of ex-skeleton Ti, TS-1 molecular sieves having good catalytic oxidation activity can hardly be obtained steadily, so the TS-1 molecular sieves obtained are generally inferior in activity stability, which handicaps the industrial application of the TS-1 molecular sieve in the prior art.
The object of this invention is to provide a novel titanium-silicalite molecular sieves (TS-1) with MFI structure, particularly having a unique morphology of crystallite and showing good catalytic reactivity and stability in oxidations. Another object of the invention is to provide a method for the preparation of the said titanium-silicalite molecular sieve.
The titanium-silicalite molecular sieve provided by the invention is characterized by crystallites with hollow struture, in which the hollow cavity of each crystallite has a radial length in the range of 5xcx9c300 nm, preferable 10xcx9c200 nm. The benzene adsorption capacity of said titanium-silicalite sample tested at 25xc2x0 C. and P/P0=0.10 for 1 h is at least 70 mg/g, preferably at least 80 mg/g.
Said titanium-silicalite molecular sieve provided by the invention is also characterized by that the cavity shape of said titanium-silicalite crystallite can be varied, such as in circular, or rectangular, or irregularly polygonal, or irregularly circular, or a combination of these shapes.
The grains of the titanium-silicalite molecular sieve with MFI structure provided by the invention are composed of individual hollow crystallites or aggregated crystallites as a result of aggregation of the hollow crystallites.
Said titanium-silicalite molecular sieve provided by the invention is also characterized by that there is an obvious hysteresis loop between the low-temperature N2 adsorption isotherm and desorption isotherm of said molecular sieve, while generally there is not any hysteresis loop between those isotherms of the conventional titanium-silicalite. The applicants in their research found that the hysteresis loop is related to the hollow cavity structure in the crystallites of said titanium-silicalite molecular sieve. The bigger the hollow cavity in the crystallites, the larger the said hysteresis loop.
The SiO2: TiO2 molar ratio of said titanium-silicalite molecular sieve of the invention ranges from 5 to 500, preferably from 10 to 200.
The first method for the preparation of said titanium-silicalite molecular sieve provided by the invention comprises the following steps:
(1) Mixing a sythesized TS-1 with an acidic compound and water homogenously, and letting the mixture react at 5-95xc2x0 C. for 5-360 min., preferably at 15-60xc2x0 C. for 10-180 min. to obtain an acid-treated TS-1;
(2) Mixing the acid-treated TS-1 obtained in step 1 with an organic base and water homogenously, then transferring the mixture to a autoclave to react at 120-200xc2x0 C. under autogenous pressure for 1-192 hr., preferably at 150-180xc2x0 C. under antogenous pressure for 2-120 hr, then filtrating, washing, drying and calcining the resultant to obtain the TS-1.
In the first method according to the present invention, step 1 and step 2 may be repeated once or several times respectively, or only step 2 is repeated once or several times, so as to further enlarge the hollowness in the crystallites, reduce the amount of ex-skeleton TiO2 and improve the catalytic reactivity of the TS-1 molecular sieve.
Said TS-1 used in step 1 of the first method of the present invention can be a synthesized TS-1 prepared by any method of the prior art, which can be as-synthesized or calcined, i.e. with or without an organic template, preferably calcined and with the organic template removed.
Said acidic compound used in step 1 of the first method is selected from organic acid compounds, such as an aliphatic acid R1(COOH)n, wherein R1 is a C1-4-alkyl, and n is 1 or 2; or inorganic mineral acids; such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid and hydrofluoric acid; or acidic salt compounds, such as ammonium chloride, ammonium phosphate, ammonium nitrate, ammonium sulfate and ammonium fluoride; preferably inorganic acid.
The ratio of molecular sieve to acidic compound to water used in step 1 of the first method is: molecular sieve (g): acidic compound (mol): water (mol)=100:(0.010-2.0):(5-250), preferably 100:(0.080-0.80):(10-100).
The organic base used in step 2 of the first method of the present invention is selected from the group consisting of aliphatic amines, alkylol amines, quaternary ammonium bases, or mixtures of these compounds, preferably alkylol amines, or quaternary ammonium bases or mixtures of these compounds; most preferably quaternary ammonium bases or mixtures of the organic base and quaternary ammonium bases.
Said aliphatic amines have a general formula of R2(NH2)n, wherein R2 is a C1-6-alkyl, and n is 1 or 2, preferably ethyl amine, n-butyl amine, butanediamine, or hexanediamine.
Said alkylol amine compounds have a general formula of (HOR3)mN, wherein R3 is a C1-4-alkyl, and m is from 1 to 3, preferably mono ethanolamine, diethanolamine or triethanolamine.
Said quaternary ammonium bases have a general formula of R44NOH, wherein R4 is a C1-4-alkyl, preferably a C2-4-alkyl, more preferably propyl.
The ratio of molecular sieve to organic base to water used in step 2 of the first method is: molecular sieve (g): organic base (mol.): water (mol)=100:(0.005-0.50):(5-200), preferably 100:(0.010-0.15):(20-80).
The second method for the preparation of said titanium-silicalite (TS-1) molecular sieve according to the present invention is characterized in that the method comprises that a synthesized TS-1 molecular sieve having a MFI structure prepared by conventional method is mixed with an organic base and water homogenously, then the mixture is transferred to a sealed reaction vessel to react at 120-200xc2x0 C. under autogenous pressure for 1-192 h, preferably at 150-180xc2x0 C. for 2-120 h, and then the reaction product is recovered. Compared with the first method, the second method is different in that it saves the step of acid treatment and treats the above-said synthesized TS-1 molecular sieve having a MFI-structure with an organic base directly, wherein said organic base and its amount used are the same as the afore-mentioned. In the second method, the organic base treatment can be repeated once or several times.
The hollow cavity in the crystallite of said TS-1 molecular sieve product obtained by the first method is larger than that produced by the second method, while the second method can also achieve the object of the invention.